This general type of device is often used with tools which have several cavities for simultaneous molding of several pieces of goods which are usually identical, but may also be different. It is desirous both to reduce the amount of material wasted as much as possible, and to attain the shortest possible injection molding cycle time. Molds of this kind may be designed with a common inlet channel, a sprue, which branches into small runners, one for each cavity. In order to reduce the flow length in cold steel, the sprue may be substituted by a heated sprue bushing. Further improvements may be achieved by distributing the flow of material to two or more sprue bushings in a hot channel manifold. The best result is achieved if there is an individual sprue bushing for each single mold cavity. The sprue bushing may include, among other features, a front piece and a heating element.
In order to keep the cycling times as short as possible and to avoid overheating of the plastic material, it is desired that the temperature of the material be maintained steady and as low as possible. This is partly attained by the above described sprue bushings which are heated by electrical resistance elements on the outside of each. Temperature conditions are attained and control of the elements is by means of thermocouples or other temperature sensors. Further improvement is achieved by insulating the sprue bushings from surrounding metallic materials. Ceramic insulating material has proven to be suitable.
When injection molding using the above mentioned methods, it is essential that the temperature of the material at the inlet to the mold cavity be high enough so that the material will flow into and spread within the mold cavity, without being so high that the material will be damaged. At the same time it is desirous to keep the temperature low enough so that the material will solidify rapidly inside the mold so that the product may be discarded and another cycle of operation started.
The temperature at the tip of the ingot will then be crucial.
An object of the present invention is a means of sprue bushings which will further improve the possibilities to reduce the transfer of heat from the thermoplastic material to the material in and at the tip of the bushing. It is a further object to reduce the wear of the material of the bushing.